I'm curious - where (which airports) have you had the upgrade processed prior to your request, and do you have a sense of the upgrade priority?

I took a week off from NW/Delta and flew FL this week.

10/5/09 342 MKE to SFO - Checked at the gate 40 minutes prior to departure; no BC available.
10/9/09 343 SFO to MKE - Called the elite line at 12:01am EDT to do the 4 credit upgrade and was told that BC was full.

I am now convinced that my FL travels are best served by short-haul routes where most PAX are too cheap to pay the $49 upgrade fee.